1. Field of the Invention
The present invention relates to a starch-based coating composition. In particular, the invention relates to a starch-based coating composition utilizing starch which is naturally derived and biodegradable, having excellent storage stability as a one-pack type paint, and capable of forming coating films with excellent drying properties, finished appearances, pencil hardness, mar resistance, adhesion, alkali resistance, solvent resistance and weather resistance, as well as to coated articles coated with the starch-based coating composition.
2. Description of Related Art
Recent years have seen an increased demand for active use of naturally derived, biodegradable raw materials with a low environmental load, from the standpoint of minimizing effects on the earth environment by improving waste treatment and lowering CO2 emissions.
Typical naturally derived materials include modified starches such as polysaccharide starches or acetylated starches which have conventionally been used in the food and papermaking industries, but recently such starches have come into use as biodegradable plastic materials in the form of products for a wide range of fields including food containers, packaging materials, buffer material sheets, agricultural films, disposable diapers and the like.
Starch that has been subjected to various types of processing to partially modify its original structure or properties is called “chemically modified starch”, and starches have been modified and improved in various ways by chemical modification for utilization as starting materials for industrial products. The basic structure of starch is a mixture of amylose consisting of α-D-glucose linked in a linear fashion by 1,4-bonds and amylopectin with the same in a branched structure, and modifications such as esterification and etherification utilizing the hydroxyl groups in the structure have been employed since the 1960s.
There have also been proposed urethanated starches wherein at least some of the hydroxyl groups of starch or modified starch are urethanated by reaction with isocyanate compounds (see Japanese Unexamined Patent Publication No. 5-43649).
There has further been proposed a process for production of biodegradable polyurethane by reaction of a polyisocyanate with an organic solvent solution containing at least one type of plant component selected from among hydroxyl-containing modified starches or modified starches, molasses, polysaccharide-based agricultural wastes and vegetable oils (see Japanese Unexamined Patent Publication No. 5-186556).
Similarly, bonding of starches and hydroxyl-containing acryl resins with polyisocyanates has also been proposed (see Japanese Unexamined Patent Publication No. 6-65349).
This involves indirect grafting of the “starch resin” and acryl resin via the polyisocyanate, but several publications have also disclosed methods of direct production of graft starches obtained by radical graft polymerization of unsaturated monomers with starches or modified starches (see J. C. Arthur, Jr.; Advan. Macromol. Chem.; “Graft Polymerization onto Polysaccharide”; 2:1-87(1970), U.S. Pat. No. 3,425,971, U.S. Pat. No. 3,981,100, Japanese Unexamined Patent Publication No. 54-120698, Japanese Unexamined Patent Publication No. 55-90518, Japanese Unexamined Patent Publication No. 56-167746, Japanese Unexamined Patent Publication No. 8-239402).
In addition, as examples of combinations of starch with other biodegradable resins, inventions have been disclosed that employ as molding materials different polymer blends comprising combinations of starch or modified starch with cellulose derivatives (see Japanese Unexamined Patent Publication No. 6-207047, Japanese Unexamined Patent Publication No. 8-231762).
As clearly taught in these publications, starch-based resins with combinations of different polymers either bonded or grafted are known. However, all of these conventional techniques assume that the uses of the starch-based resins are for structural materials, injection molding materials, sheets and the like, whereas no uses as paints have been disclosed.
For coating using a starch-based resin, there has been disclosed the use of a reactive curing paint which is a curable starch composition comprising a mixture of a starch-based resin and a curing agent having a functional group that reacts with the multiple hydroxyl groups in the starch molecule (see Japanese Unexamined Patent Publication No. 2004-224887).
There has also been disclosed a water-dispersed resin with a mean particle size of no greater than 1000 nm comprising as a constituent component a copolymer of (A) modified starch and (B) a polymerizable unsaturated monomer, and the use of a water-based coating composition containing the resin, as a reactive curing paint (see Japanese Unexamined Patent Publication No. 2006-52338).
However, this starch-based paint technology of the prior art is concerned with reactive curing paints, whereas a one-pack lacquer type starch-based paint has not yet been developed that exhibits excellent storage stability and is capable of forming a coating film with superior drying property, finished appearances, pencil hardness, mar resistance, adhesion, alkali resistance, solvent resistance and weather resistance.